1. Field of the Invention
The present invention relates to a method for operating a magnetic resonance tomography apparatus of the type having a shim coil arrangement and a gradient coil arrangement, whereby, in order to generate an image dataset of a region of an examination subject, at least the region that must be imaged is borne in an imaging volume of the device, and an initial shim adjustment procedure is carried out.
2. Description of the Prior Art
In magnetic resonance tomography, the homogeneity of the base magnetic field is a decisive factor with respect to the quality of the magnetic resonance images. Field inhomogeneities of the base magnetic field within the imaging volume of a magnetic resonance tomography system cause geometric distortions of the magnetic resonance image that are proportional to the field inhomogeneities. Field homogeneity is particularly important in sequences known as rapid pulse sequences, such as in echo-planar methods.
Measures to improve the base magnetic field homogeneity are referred to as shimming. There are passive and active shim measures. In passive shimming, a number of iron plates are attached in the examination space of the magnetic resonance tomography system in an appropriate arrangement. The base magnetic field in the imaging volume is measured prior to attaching the iron plates. From the measured values, a computer program calculates the appropriate number and arrangement of the iron plates.
In active shimming, correction coils which homogenize the base magnetic field, known as shim coils, are provided in a shim coil arrangement. To operate the shim coil arrangement, a power pack that delivers highly constant and reproducibly adjustable direct currents is needed. The shim coil arrangement is used for fine correction whenever a very high homogeneity is needed, for instance to correct field distortions that are caused by the susceptibility of an examination subject.
As is known from German Patent 195 11 791, corresponding to U.S. Pat. No. 5,614,827, the base magnetic field in the imaging volume can be described by coefficients of spherical harmonic functions. This patent also teaches that linear base magnetic field deviations, i.e. first-order field disturbances, can be compensated by charging gradient coils with an offset current. The offset current is a constant current that is superimposed on a current that executes a gradient sequence. To compensate higher-order field inhomogeneities, respective shim coils are provided in correspondence to the order that must be compensated, each such coil essentially compensates the corresponding field disturbance, for which it must be charged with a suitable current. In magnetic resonance tomography, nine shim coils generally suffice even given high requirements, so that, together with the three gradient coils, twelve spherical coefficients that disturb the field homogeneity most severely can be cancelled.
Due to the field-distorting effect of the examination subject, a shim adjustment procedure is performed in the course of generating magnetic resonance images. In this process, the currents for the individual shim coils and the offset currents for the gradient coils are determined once subsequent to positioning the volume of a region of the examination subject that is to be imaged. The shim adjustment procedure is carried out according to the following steps in accordance with the abovementioned patent:
In a first step, two three-dimensional, spatially resolved magnetic resonance raw datasets are defined in the form of two three-dimensional raw data matrices whose phases have different sensitivities to inhomogeneities of the base magnetic field. The first raw data matrix is obtained using a first series of sequences with a first echo time. The same series of sequences is repeated with a second echo time which is larger than the first echo time. The second raw data matrix is obtained therefrom. In the second raw data matrix, base magnetic field inhomogeneities influence the phase of the measured signals more strongly, since the base magnetic field inhomogeneities have a longer effect longer due to the longer echo time.
In a second step, the two raw data matrices undergo a three-dimensional Fourier transformation.
In a third step, a three-dimensional phase difference matrix is calculated by determining phase differences between corresponding voxels of the two Fourier transformed matrices.
In a fourth step, phase differences between spatially adjacent voxels are calculated in the phase difference matrix PD. This is done for all three spatial directions, with a phase error dataset being produced for each direction, for example.
In a fifth step, the currents for each shim coil and for each gradient coil are computed based on the measured phase error dataset and on a predetermined matrix A. The matrix A characterizes the effect of one unit current on each voxel of the phase error dataset for each shim coil and for each gradient coil. This matrix A must be defined only once for each magnetic resonance tomography system and then remains constant as long as there are no modifications to the system.
In a sixth and final step, the computed shim currents in the shim coil arrangement and the offset currents in the gradient coils are correspondingly adjusted.
The computed shim setting remains unchanged for all magnetic resonance images of the examination subject that must be obtained. Apart from the one-time determination of the matrix A, the first step of the initial shim adjustment process described above is by far the most time consuming. Even when, instead of the first and second series of sequences, only one sequence is used, which delivers two signals with different echo times following a single excitation, and in addition a substantially smaller spatial resolution is selected compared to a diagnostic magnetic resonance image, the time frame for step one is always in the 30-second range.
Subsequent to a position change of the imaged region, for instance as a result of a movement of the examination subject, the shim setting that was calculated in the initial shim adjustment procedure is no longer optimal, and consequently the images that are created in the continued course of the magnetic resonance tomography procedure contain distortions dependent upon the position modification. Such distortions resulting from a change of position of the imaged region can be avoided by undertaking a new shim adjustment procedure as described above. For this purpose, the magnetic resonance data acquisition of the examination subject would have to be interrupted, so the time-consuming shim adjustment procedure can be performed again. This would lead to an unacceptable prolonging of the duration of the magnetic resonance tomography procedure, particularly given repeated position changes, and it is therefore not done in practice.
It is an object of the present invention to design a method of the abovementioned type with which it is possible to achieve an optimal shim adjustment at any time during a magnetic resonance tomography procedure in a time-efficient manner, even when the imaged region changes its position.
This object is inventively achieved in a method for operating a magnetic resonance tomography apparatus wherein a position change of the imaged region in relation to the imaging volume is detected, and a current in the shim coil arrangement is modified dependent on the detected position change.
For simplifying the description of the advantages of the inventive method, it is assumed, without limiting the method, that the base magnetic field in the imaging volume is sufficiently homogenous without an examination subject therein, and that a region of the examination subject that must be imaged is a head of a patient. Subsequent to positioning the head in the imaging volume of a magnetic resonance tomography device, an initial shim adjustment procedure is carried out as described above. With this procedure, the distorting effect of the head on the homogeneity of the base magnetic field (which is presumed to be sufficiently homogenous) is detected, and the currents in a shim coil arrangement are adjusted so as to counteract the aforementioned distortion. Each change of the position of the head, for instance due to a movement by the patient, leads to a change of the homogeneity characteristics in the imaging volume, so that the initially calculated shim setting is no longer optimal, leading to corresponding image distortions. For this reason, if a change of position of the head occurs, this is inventively detected, and the currents in the shim coil arrangement are adapted according to the detected change of position. In this way, a high image quality is guaranteed for all magnetic resonance images. The adjustment of the shim coil currents is accomplished in a time-efficient manner by means of the distorting effect of the head that was calculated in the initial shim adjustment procedure simply being transferred to a new position or orientation of the head in the imaging volume. A time-consuming sequence such as the above described step one of the conventional initial shim adjustment procedure need not be performed. In the context of the above described method for shim adjustment from U.S. Pat. No. 5,614,827, the phase difference matrix is shifted in accordance with the inventive method, i.e. rotated, in correspondence to the detected translation, i.e. rotation, of the head, and is fed into the remaining shim adjustment procedure at step four as the new phase difference matrix.
The inventive method also can be used in base magnetic fields that are more or less inhomogeneous without an examination subject therein. In addition to the above described inventive method steps, it is then necessary to know the homogeneity distribution of the base magnetic field in the imaging volume without an examination subject present. For example, the distorting effect of the head plus the inhomogeneity of the base magnetic field without an examination subject can be compensated in an initial shim adjustment procedure. Given knowledge of the aforementioned homogeneity distribution and knowledge of the position of the head in the imaging volume, the distorting effect that is caused solely by the head can be computed. Given a change of the position of the head, the shim coil currents are adjusted by transferring the distorting effect to the new position, i.e. orientation, of the head, and the known homogeneity distribution at the new position is additionally taken into consideration.
In an embodiment of the inventive method, an offset current in the gradient coil arrangement is adapted in correspondence to the detected position change. Only by resetting the offset currents in the gradient coils in connection with a resetting of the shim currents does a complete adaptation of the shim setting take place.
In another embodiment the position change is computed from image datasets that are generated successively in time. As a result, additional devices or sequences for detecting position changes are not needed. As described above, position changes of the region that must be imaged lead to image distortions. Therefore, in order to detect position changes reliably, the resulting image distortions in the magnetic resonance image must be sufficiently small in relation to the position changes. This condition is satisfied, particularly in the case of larger position changes, such as rotations of the head by more than 5 degrees. This has the beneficial effect that, for example, given large rotations the rotation is reliably calculated from the image datasets, and in correspondence to the large position change a relatively large distortion effect is compensated.
A group of methods for calculating position change from image datasets that are picked up successively in time is based on describing an arbitrary rigid body movement in three-dimensional space using six movement parameters, three of which characterize translations and three of which characterize rotations. These parameters are entered in a column vector {right arrow over (q)}. The values of all voxels or select voxels of a first image dataset and a second image dataset that was generated subsequent to the first dataset are entered in a matching order in a first column vector x and a second column vector y . To calculate a position change between the pickup times of the first and second image datasets, i.e. to determine the movement parameters, the following equation, which is based on a first-order Taylor embodiment, is solved, for instance by an iterative method:                               y          →                -                  x          →                    =                                                  [                                                                                                                  ∂                                                  x                          1                                                                                            ∂                                                  q                          1                                                                                                                          ⋯                                                                                                      ∂                                                  x                          1                                                                                            ∂                                                  q                          6                                                                                                                                                          ⋮                                                        ⋰                                                        ⋮                                                                                                                                      ∂                                                  x                          n                                                                                            ∂                                                  q                          1                                                                                                                          ⋯                                                                                                      ∂                                                  x                          n                                                                                            ∂                                                  q                          6                                                                                                                                ]                        ·                          q              →                                ⁢                      xe2x80x83                    ⁢          with          ⁢                      xe2x80x83                    ⁢                      x            →                          =                  [                                                                      x                  1                                                                                    ⋮                                                                                      x                  n                                                              ]                      ;                      y        →            =              [                                                            y                1                                                                        ⋮                                                                          y                n                                                    ]              ;                  q        →            =              [                                                            q                1                                                                        ⋮                                                                          q                6                                                    ]            
More detailed description of position change detection algorithms of the above group which are based on image datasets, are provided in the book xe2x80x9cHuman Brain Functionxe2x80x9d, by R. S. J. Frackowiak (Academic Press, 1997, ch. 3, pp. 43-58) and the article xe2x80x9cMovement-Related Effects in fMRI Time Seriesxe2x80x9d (K. J. Friston, Magnetic Resonance in Medicine 35, 1996: 346-355).
In another group of methods for position change detection based on image datasets, all or selected points of a first image dataset that is described in k space, and of a second image dataset that was generated subsequent to the first dataset are compared to each other. From this comparison, a position change can be computed in a similar manner as with navigator echoes, which will be described below. These methods are described in detail in the article xe2x80x9cDecoupled Automated Rotational and Translational Registration for Functional MRI time Series Data: The DART Registration Algorithmxe2x80x9d (L. C. Maas; Magnetic Resonance in Medicine 37; 1997:131-139) and in the article xe2x80x9cSymmetric Phase-Only Matched Filtering of Fourier-Mellin Transforms for Image Registration and Recognitionxe2x80x9d (Q. Chen, IEEE Transactions on Pattern Analysis and Machine Intelligence 16; 12; 1994: 1156-1168).
In a further embodiment the image datasets are two-dimensional datasets. From two-dimensional image datasets that are generated successively in time, it is possible to compute position changes rapidly and easily.
In another embodiment, the image datasets are three-dimensional image datasets. From three-dimensional image datasets that are generated successively in time, it is also possible to compute complex position changes in all three directions in space as well as rotations.
In another embodiment the image datasets are generated by an echo-planar method. In this way, a rapid generation is achieved particularly of large three- dimensional image datasets. Besides echo-planar methods, methods of similar speed such as RARE, HASTE and GRASE can be used.
In a further embodiment, the position change is computed by an orbital navigator echo. An orbital navigator echo is a magnetic resonance signal that is characterized by a circular path in k space and that is generated by a specific navigator sequence. Using orbital navigator echoes that are generated at different times, position changes, i.e. rotations and translations, of a slice can be computed within a plane that is spread by the slice. To this end, before an image dataset of the slice is picked up by means of an imaging sequence, the navigator sequence is executed, and a navigator echo is picked up, which is used as a reference. After a definite period of time, an additional image dataset of the slice is generated, with an additional preliminary navigator echo pick-up. By comparing the two navigator echoes, a position change of the slice that occurs between the two navigator echoes can be detected. Specifically, given a rotation of the slice, the magnitude of the second navigator echo changes relative to the navigator echo serving as the reference, and given a translation in one or both directions of the planes, the phase thereof changes. Arbitrary position changes in three-dimensional space are detected by means of multiple orbital navigator echoes. This is described in the article xe2x80x9cReal-Time Prospective Correction of Complex Multiplanar Motion in fMRIxe2x80x9d (H. A. Ward, Proc. Of ISMRM 7; 1999: 270). It is also possible to detect small position changes using orbital navigator echoes, without having to provide additional devices at the magnetic resonance tomography system for this purpose.
In another embodiment the position change is detected optically. In this way, even the smallest position changes can be reliably detected independently (in terms of time) of the imaging sequences of the magnetic resonance tomography system that must be executed. For example, in a version of this embodiment, optical reflectors are attached at the region that must be imaged, the positions and orientations of which are captured by a stereoscopic camera. The aforementioned detection device together with the associated method for operating the device are described in the U.S. Pat. Nos. 5,828,770 and 5,923,417.
In a further embodiment, the position change is detected at a time interval of from 0.1 to 5 seconds. In this way, a reliable detection of position changes is achieved. In functional magnetic resonance tomography procedure in particular, the time interval is advantageously selected in accordance with the time cycle with which image datasets are generated in the context of the particular functional magnetic resonance tomography procedure.